Standing on stage before an audience of screaming Japanese teenagers, Cherie Currie (Dakota Fanning), the willowy, feather-haired lead singer of the Runaways, knits her eyebrows into a stare of defiance and spits out the lyrics to the band’s wild-girl anthem. ”Hello, Daddy! Hello, Mom!” she sings. Then, twitching her shoulders back and forth in sexy machine-gun time to the music, she snarls: ”I’m your ch-ch-ch-ch-ch-ch-cherry bomb!” She’s not a great singer, but what a stance, a ‘tude, an image!

In the mid- to late ’70s, the Runaways, a packaged group of choppy-haired teen-glam feline punkettes from L.A., ? did for girls playing power chords what the Sex Pistols did for beer-spewing anarchy. Not that they were very popular. (They were huge in Japan?and that’s about it.) If anything, the Runaways seem cooler now than they did then; it’s only in hindsight that they come off as fearless trailblazers rather than as a novelty act. The most entertaining thing about The Runaways, a highly watchable if mostly run-of-the-mill group biopic, is that its writer-director, Floria Sigismondi, has a sixth sense for how the Runaways were bad-angel icons first and a rock & roll band second.

Early on, we see Kristen Stewart, as the black-shag-haired Joan Jett, in an L.A. boutique, where she has to coerce the saleswoman ? into selling her a man’s studded biker jacket. Stewart’s casually likable, no-frills performance starts with Jett’s tough-girl saunter — which is to say, the actress knows just how to walk like a skinny dude. At the same time, we meet Cherie (pronounced Sher-ee), who cuts her platinum hair into a David Bowie shag so that she can lip-synch to him at a high school talent contest.

These girls have their outlaw fashion bona fides down. But it takes Kim Fowley, the L.A. record producer who becomes their shrewd, hectoring Svengali, to teach them how to rock out like boys. Fowley, who favors red leather jackets and dog collars the size of tiaras, is a hyped-up hustler-manipulator who has seen through the rebel artifice of rock & roll yet loves it anyway. As played by Michael Shannon, the great actor from Revolutionary Road, he looks like a punk Frankenstein and shouts everything as if in mid-tantrum. He’s a creep, and proud of it, but he knows what sells. He places Cherie in the band based on her looks alone, as if he were casting a porno film. That she’s only 15 is just icing on the bad-girl cake.

There’s a fun scene set in a grungy rehearsal trailer, where Fowley, with a little help from Joan, makes up ”Cherry Bomb” on the spot and teaches Cherie to sing it with nasty glee. You can see the girls co-opting the male hormonal thrust of rock and making it their own.

When it gets away from the stage, though, and from the iconography of strutting she-devil-in-lingerie empowerment, The Runaways is a glumly episodic rock saga. It stays true to how the band’s members were exploited, yet there’s a special challenge in bringing this story to life, since the Runaways were really just little girls who fed themselves into a buzz-saw machine of record-industry hype. They were passive vessels in their own story, and the film makes them even more passive: As portrayed, they’re so generically brash that they don’t have full-scale personalities. Stewart nails Jett’s sinewy swagger, but Joan’s lesbian proclivities are treated in a teasing, music-video way. I mean, why be so coy in a film that’s meant to be a rowdy salute to a new kind of audacious feminine sexual power?

But then, Joan is really the side player here. The Runaways is fashioned as Cherie Currie’s story — and the bite-size conflicts provided for her, while true to the facts, aren’t enough to give the film a dramatic center. She squabbles, tiresomely, with her twin sister (Riley Keough). And when Fowley forces her to do a solo cheesecake shoot, Joan chews her out for selling out the band’s image. (Does Joan realize what the band is selling?) The film shows that the Runaways were authentic — if packaged — stars who were out of their depths because they were ahead of their time. As a band, they get their due, but as individuals the movie seldom makes them more than victims. B-